Electrolytes for rechargeable batteries

ABSTRACT

Provided are novel electrolytes for use in rechargeable lithium ion cells containing high capacity active materials, such as silicon, germanium, tin, and/or aluminum. These novel electrolytes include one or more pyrocarbonates and, in certain embodiments, one or more fluorinated carbonates. For example, dimethyl pyrocarbonate (DMPC) may be combine with mono-fluoroethylene carbonate (FEC). Alternatively, DMPC or other pyrocarbonates may be used without any fluorinated carbonates. A weight ratio of pyrocarbonates may be between about 0% and 50%, for example, about 10%. Pyrocarbonates may be combined with other solvents, such as ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and/or ethyl-methyl carbonate (EMC). Alternatively, pyrocarbonates may be used without such solvents. Experimental results conducted using electrochemical cells with silicon based electrodes demonstrated substantial improvements in cycle life when pyrocarbonate containing electrolytes were used in comparison with pyrocarbonate free electrolytes.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit under 35 U.S.C. §119(e) of U.S.Provisional Patent Application No. 61/413,888, entitled “ELECTROLYTESFOR RECHARGEABLE BATTERIES,” filed on Nov. 15, 2010, which isincorporated herein by reference in its entirety for all purposes.

STATEMENT OF GOVERNMENT SUPPORT

The invention described and claimed herein was made with United StatesGovernment support under NIST ATP Award No. 70NANB10H006, awarded by theNational Institute of Standards and Technology. The United StatesGovernment has certain rights in this invention.

BACKGROUND

The energy density of lithium ion batteries can be substantiallyimproved by carbon-based electrode materials with high capacity activematerials, such as silicon. Yet, high capacity materials present a newset of challenges not previously encountered with carbon-basedmaterials. For example, the cycle life of cells built with high capacityactive materials and conventional electrolytes tends to be much shorterthan the cycle life of cells built with carbon based active materialsand the same electrolytes. The selection of electrolytes may impactformation of solid electrolyte interphase (SEI) layers, ionic mobility,and various other factors that collectively impact the cycle life of acell. Specific electrolyte formulation may be necessary to address thesenew challenges presented by introducing high capacity active materialsinto lithium ion batteries.

SUMMARY

Provided are novel electrolytes for use in rechargeable lithium ioncells containing high capacity active materials, such as silicon,germanium, tin, and/or aluminum. These novel electrolytes include one ormore pyrocarbonates and, in certain embodiments, one or more fluorinatedcarbonates. For example, dimethyl pyrocarbonate (DMPC) may be combinewith mono-fluoroethylene carbonate (FEC). Alternatively, DMPC or otherpyrocarbonates may be used without any fluorinated carbonates. A weightratio of pyrocarbonates may be between about 0% and 50%, for example,about 10%. Pyrocarbonates may be combined with other solvents, such asethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate(DMC), diethyl carbonate (DEC), and/or ethyl-methyl carbonate (EMC).Alternatively, pyrocarbonates may be used without such solvents.Experimental results conducted using electrochemical cells with siliconbased electrodes demonstrated substantial improvements in cycle lifewhen pyrocarbonate containing electrolytes were used in comparison withpyrocarbonate free electrolytes.

In certain embodiments, a lithium ion battery includes an electrodehaving an electrochemically active material, such as a siliconcontaining material, a tin containing material, a germanium containingmaterial, and an aluminum containing material. The battery also includesan electrolyte comprising a lithium containing salt and a pyrocarbonate,such as dimethyl pyrocarbonate, diethyl pyrocarbonate, dipropylpyrocarbonate, dibutyl pyrocarbonate, and ethylmethyl pyrocarbonate. Inspecific embodiments, the electrochemically active material includeselemental silicon or a silicon alloy. The electrode may includenanostructures, such as nanowires, with the electrochemically activematerial provided in these nanostructures. In certain embodiments, anouter surface of the nanowires includes elemental silicon, siliconoxide, silicon alloy, or silicide.

In specific embodiments, the battery includes silicon as itselectrochemically active material. This battery may exhibit an averageCoulombic efficiency of at least 99.8% after about 100 cycles during acycling test. The cycling test may include charging the lithium ionbattery to at least about 1050 mAh/g at rate of at least about C/2 anddischarging to 900 mV versus lithium metal at a rate of at least aboutC/2 in each cycle. In certain embodiments, the electrolyte includes oneor more pyrocarbonates that have a total concentration in theelectrolyte of less than about 50% by weight, such as between about 1%and 10% by weight. In the same or other embodiments, the electrolyteincludes one or more fluorinated carbonate solvents, such asmono-fluoroethylene carbonate, fluoropropylene carbonate,difluoroethylene carbonate, and fluoromethylethylene carbonate. A totalconcentration of the fluorinated carbonate solvents in the electrolytemay be less than about 50% by weight, for example, less than about 10%by weight.

Provided is also a lithium ion battery electrolyte for use in a lithiumion battery containing high capacity active materials. The electrolytemay include a lithium containing salt, a solvent, DMPC, and one or morefluorinated carbonate solvents. The DPMC may be present in the lithiumion battery electrolyte at a concentration of less than about 50% byweight (for example, between about 1% and 10% by weight). Examples offluorinated carbonate solvents include mono-fluoroethylene carbonate,fluoropropylene carbonate, difluoroethylene carbonate, andfluoromethylethylene carbonate. The one or more fluorinated carbonatesolvents may be present in the lithium ion battery electrolyte at aconcentration of between less than about 50% by weight, for examples,between about 1% and 10% by weight. Examples of the solvent used in thiselectrolyte include EC, DMC, DEC, and/or EMC. Examples of the lithiumcontaining salt used in this electrolyte include lithiumhexafluorophosphate (LiPF6), lithium hexafluoroarsenate monohydrate(LiAsF6), lithium perchlorate (LiClO4), lithium tetrafluoroborate(LiBF4), lithium bis(oxalato)borate (LiBOB), lithiumoxalyldifluoroborate (LiODFB), LiPF3(CF2CF3)3 (LiFAP), andLiBF3(CF2CF3)3 (LiFAB).

For purposes of this document, all concentration values of electrolytecomponents (with the exception of lithium containing salts, which arepresented as molar ratios) are presented as weight percentages unlessotherwise noted. The term “concentration” is generally usedinterchangeably with the term “weight ratio.”

These and other embodiments are described further below with referenceto the figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates Coulombic efficiency data for the silicon based testcells containing different electrolyte formulations.

FIG. 2A is schematic perspective view of a core-shell nanostructure 200that includes a core, inner shell, and outer shell, in accordance withcertain embodiments.

FIG. 2B is schematic cross-section view of nanostructure 200, inaccordance with certain embodiments.

FIG. 3 is a schematic representation of an electrode structure duringdifferent stages of its fabrication, in accordance with certainembodiments.

FIG. 4 is a schematic representation of a multidimensionalelectrochemically active structure, in accordance with certainembodiments.

FIG. 5A is a plan view of a partially-assembled electrochemical cellthat uses electrodes described herein, according to certain embodiments.

FIG. 5B is a cross-section view of an electrode stack 500 of thepartially-assembled electrochemical cell that uses electrodes describedherein, according to certain embodiments.

FIG. 6A is a schematic cross-section view of a jellyroll, in accordancewith certain embodiments.

FIG. 6B is a schematic perspective view of a jellyroll, in accordancewith certain embodiments.

FIG. 6C is a schematic cross-section view of a prismatic wound cell, inaccordance with certain embodiments.

FIG. 7A is a schematic cross-section view of a stacked cell, inaccordance with certain embodiments.

FIG. 7B is a schematic perspective view of a stacked cell, in accordancewith certain embodiments.

FIG. 8 is a cross-section view of the wound cylindrical cell, inaccordance with certain embodiments.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

In the following description, numerous specific details are set forth inorder to provide a thorough understanding of the presented concepts. Thepresented concepts may be practiced without some or all of thesespecific details. In other instances, well known process operations havenot been described in detail so as to not unnecessarily obscure thedescribed concepts. While some concepts will be described in conjunctionwith the specific embodiments, it will be understood that theseembodiments are not intended to be limiting.

The adoption of high capacity active materials for rechargeable batteryapplications has been relatively slow. One major obstacle is the poorcycle life of the electrochemical cells built with such materials. Whilethe initial capacities of these cells are generally much higher than theinitial capacity of cells built with carbon-based materials, thesehigher initial capacities rapidly decline after only a few cycles.Often, these capacities fade below a usable level after only tens ofcycles. Clearly, such cells are not usable for commercial applications.While some improvement has been achieved by specifically arranging highcapacity negative active materials into particular structures, oftenthis level of improvement is not sufficient to provide commerciallyfeasible cell design by itself.

Without being restricted to any particular theory, it is believed thatthe poor cycle life of high capacity active materials may beattributable to at least two main causes. One of these causes is poormechanical stability of high capacity active material structures duringcycling, which causes pulverization of these structures. Many highcapacity active materials tend to exhibit substantial swelling duringlithiation and contraction during delithiation. For example, siliconstructures can swell about 400% when lithiated to the theoreticalcapacity of silicon. Pulverization of the active material structuresresults in losses of mechanical and electrical connections within theelectrode, which contribute to capacity fading.

Another perceived cause of capacity fading in cells containing highcapacity active materials is an unstable SEI layer that typically formson the exposed surfaces of active material structures. Specifically,electrolyte components, such as solvents, tend to decompose (or reduce)on the surface of these structures when subjected to a high voltagepotential. The decomposition products are deposited on the surface,forming an SEI layer. The SEI layer contributes to and increases theelectronic resistance of the electrode as the SEI layer thickens duringadditional formation. This, in turn, reduces the voltage potential onthe exposed electrode surface and eventually stops further decomposition(reduction) and SEI layer formation. The SEI layer plays an importantrole in cell performance. As such, SEI layer formation is usuallyperformed during fabrication of the cell using tightly controlledprocess parameters, such as charge and discharge rates, depths of chargeand discharge, and other parameters.

When high capacity active materials are used, repeated swelling andcontraction of the high capacity active material structures are believedto damage the SEI layer formed on the surface of the active materialexposed to the electrolyte. The damaged SEI layer may expose some highcapacity active material maintained at a high potential to theelectrolyte, thereby causing additional decomposition of electrolyte atthis interface and the formation of a new SEI layer or patch. Theprocess of breaking and repairing the SEI layer may continue long afterthe initial formation performed during cell fabrication. A newly formedSEI layer or patches may be formed at less desirable and controlledoperating conditions. Further, repeating this breaking and repairingprocess may irreversibly trap lithium in the SEI layer, reduceconcentration of certain solvents in the electrolyte, and unnecessarilythicken the overall SEI layer resulting in a higher overall resistance.These phenomena may negatively impact cell performance and reduce thecell's cycle life.

Cycle life of a cell may be characterized by Coulombic efficiencies ofindividual lithiation and delithiation cycles. For purposes of thisdocument, a Coulombic efficiency is defined as a ratio of the dischargecapacity to the charge capacity in the same cycle. In a negativeelectrode example, a Coulombic efficiency is a ratio of a charge passedduring lithium removal from the active material structures to a chargerequired to insert lithium into these active material structures. Forpractical battery applications, a Coulombic efficiency should approachthe 100% level, such as at least about 99.5%, at least about 99.8%, oreven greater, and stay at that level during most of the operating lifetime of the cell. Any deviation from the 100% level may aggregate over alarge number of cycles and result in substantial capacity fading.

Carbon based electrodes form a stable SEI layer relatively quickly.Usually, only one or a few initial formation cycles are needed for SEIformation. This initially formed SEI layer remains generally unchangedduring later operating cycles. Various cyclic carbonates, such as EC andPC, and linear carbonates, such as DMC, DEC, and EMC, have beensuccessfully used in combination with carbon based electrodes.

Unlike carbon based active materials, high capacity active materialsexhibit substantially different behavior when combined with conventionalelectrolytes containing the cyclic and/or linear carbonates listed abovewhen no other solvents or additives are present. As explained above, itis believed that SEI layers formed over high capacity active materialstructures tend to be much more unstable than SEI layers formed overcarbon based structures. High capacity active material structures swelland contract and apply substantial stresses on relatively hard andinflexible layers formed by these cyclic and/or linear carbonates. TheSEI layers break and self-repair causing all kinds of performanceproblems, including cycle life fading, that are generally not present incells built with carbon based active materials.

While pulverization of high capacity active material structures may havebeen addressed, at least in part, by arranging these materials intonanostructures, such as nanofilms, nanowires, and nanoparticles, eventhese smaller structures are believed to generate substantial stressesduring their swelling and contraction. These stresses can damage SEIlayers formed using conventional electrolyte formulations. In fact,these smaller structures may even amplify some SEI related issues.Specifically, smaller structures tend to have much larger exposedsurface areas and have sharper angles between these structures thantheir bulkier counterparts. These characteristics of smaller structuresmay lead to even larger and less stable SEI layer.

It has been found that certain new electrolyte formulationssubstantially improve the cycle life performance of rechargeableelectrochemical cells containing high capacity active materials. Forexample, one or more pyrocarbonates may be included in an electrolyte.Test results have clearly demonstrated that the addition of DMPC to acombination of EC and DMC substantially improves a cell's cycle life.The cycle life was characterized by coulombic efficiencies of multipleinitial cycles. Some improvements have been achieved by adding FEC to acombination of EC and DMC. The best performance has been achieved whenboth DMPC and FEC have been combined with EC and DMC. These test resultsare described below in more detail.

Pyrocarbonates are believed to improve the stability and performance ofan SEI layer and may reduce capacity fading. Lower lithium dendriteformation and improved rate performance have been attributed to thepresence of pyrocarbonates in electrolytes. Furthermore, electrolytescontaining pyrocarbonates tend to produce carbon dioxide, particularlywhen pyrocarbonates are present in large concentrations. The release ofcarbon dioxide that may be particularly significant during overchargemay be used to trip safety devices, such as current-interrupting circuitbreakers. Furthermore, released carbon dioxide may act as flameretardant, thereby contributing to overall cell safety during runawaysituations.

In certain embodiments, one or more pyrocarbonates are combined with oneor more cyclic and/or linear carbonates in the same electrolyte. Inother embodiments, one or more pyrocarbonates may completely replacelinear carbonates. That is, such electrolyte formulations aresubstantially free from linear carbonates.

Various alkyl pyrocarbonates may be employed in electrolytes. Generally,the pyrocarbonate functional group has the following formula, where R1and R2 may be the same or different moieties, typically each having oneto six carbon atoms.

R1 and R2 may have some degree of unsaturation (e.g., one or more doubleor triple bonds). Further, R1 and R2 may have one or more substituents,such as halogen containing moieties or, more specifically, fluorinecontaining moieties, nitrogen containing moieties (e.g., amines andamides) and/or oxygen containing moieties (e.g., hydroxyl, ketones,aldehydes, and ethers). Still further, R1 and R2 together may form aring structure with or without any substituents.

In the same or other embodiments, one or more pyrocarbonates are used ina combination with one or more fluorinated carbonates, such asfluoroalkyl carbonates. In some specific embodiments, DPMC is used withFEC or some other one or more fluorinated carbonates. Other electrolytecomponents in these embodiments may include EC, DMC, DMC, and/or EMC. Incertain embodiments, a combination of one or more pyrocarbonates and oneor more fluorinated carbonates may completely replace cycling and/orlinear carbonates. That is, such electrolyte formulations aresubstantially free from cycling and/or linear carbonates. In certainembodiments, fluorinated pyrocarbonates are used. For example, afluorinated version of dimethyl pyrocarbonate may be used, wheretrifluoromethyl (CF3) or other similar groups represent R1 and R2 in thestructure presented above.

A weight ratio of one or more pyrocarbonates in the electrolyteformulation may be up to about 50% or, more specifically, up to about20% or, even more specifically, between about 1% and 10% such as betweenabout 2% and 5%. Similar weight ratios are applicable to fluorinatedcarbonates, such as FEC. In a particular embodiment, about 5 weight % ofFEC and about 10 weight % of DPMC are used in a combination with EC andDMC, whereas EC and DMC may be present at equal weight ratios. Inanother embodiment, about 10 weight % of FEC and about 2 weight % ofDPMC are used in a combination of EC and DMC that may be present atequal weight ratios. In yet another embodiment, about 10 weight % of FECand about 5 weight % of DPMC are used with EC and DMC that may bepresent at equal weight ratios.

Specific electrolyte formulations will now be discussed in more detail.In certain embodiments, an electrolyte includes DMPC present at thefollowing weight ratios: between about 0% and 50% or, more particularly,between about 1% and 20% or, even more particularly, between about 5%and 10% or between about 1% and 10%. Other pyrocarbonates may be used inaddition to or instead of DMPC and be present at the same weight ratios.Some examples include diethyl pyrocarbonate, dipropyl pyrocarbonate,dibutyl pyrocarbonate, and ethylmethyl pyrocarbonate.

As mentioned above, the electrolyte may also include FEC. FEC may beused in a combination with DPMC and/or one or more other pyrocarbonatesor without any pyrocarbonates. The weight ratio of FEC in theelectrolyte may be between about 0% and 50% or, more particularly,between about 1% and 20% or, even more particularly, between about 5%and 10% or between about 1% and 10%. Other fluorinated carbonates may beused in addition to or instead of FEC. Some examples includefluoropropylene carbonate, difluoroethylene carbonate, andfluoromethylethylene carbonate. Other fluorinated species, such asfluorinated cyclic carboxylates and fluorinated cyclic ethers, may beused in various combinations with or instead of fluoropropylenecarbonates.

In specific embodiments, FEC and DPMC may be used together in thefollowing combinations: between about 2 weight % and 10 weight % of FECand between about 5 weight % and 15 weight % of DPMC, or morespecifically between about 3 weight % and 8 weight % of FEC and betweenabout 8 weight % and 12 weight % of DPMC, or even more specificallyabout 5 weight % of FEC and about 10 weight % of DPMC. In otherembodiments, weight ratios of FEC and DPMC are between about 5% and 15%for FEC and between about 0.5% and 5% for DPMC, or more specificallybetween about 8% and 12% for FEC and between about 1% and 4% for DPMC,or even more specifically about 10% for FEC and about 2% for DPMC. Inyet another embodiment, weight ratios of FEC and DPMC are between about5% and 15% for FEC and between about 1% and 10% for DPMC, or morespecifically between about 8% and 12% for FEC and between about 3% and8% for DPMC, or even more specifically about 10% for FEC and about 5%for DPMC. The combinations of FEC and DPMC listed above may be usedtogether with EC and DMC that are present in the electrolyte solution atequal weight ratios. In other embodiments, FEC and DPMC are used withoutother solvents and may be present at weight ratios of between about 10%and 90% each or, more specifically, at weight ratios of between about20% and 80% each.

In certain embodiments, an electrolyte includes DMPC or a combination ofFEC and DPMC. Such electrolyte formulations may also include one or moreadditional solvents. Some examples of such additional solvents includevarious cyclic carbonates, such as EC and PC, and linear carbonates,such as DMC, DEC, and EMC. Other examples include butylene carbonate(BC), vinylethylene carbonate (VEC), gamma-butyrolactone (GBL),gamma-valerolactone (GVL) alpha-angelica lactone (AGL), methyl propylcarbonate (MPC), dipropyl carbonate (DPC), methyl butyl carbonate (NBC)and dibutyl carbonate (DBC), tetrahydrofuran (THF),2-methyltetrahydrofuran, 1,4-dioxane, 1,2-dimethoxyethane (DME),1,2-diethoxyethane, 1,2-dibutoxyethane, acetonitrile, adiponitrile,methyl propionate, methyl pivalate, butyl pivalate and octyl pivalate,amides, dimethyl formamide, trimethyl phosphate, and trioctyl phosphate.

An electrolyte may include one or more salts selected from the followinglist: LiPF₆, LiAsF₆, LiClO₄, LiBF₄, LiBC₄O₈ (LiBOB), LiBC₂O₄F₂ (LiODFB),LiPF₃(C₂F₅)₃ (LiFAP), LiBF₃(C₂F₅) LiPF₃(C₂F₅)₃ (LiFAB), LiN(CF₃SO₂)₂,LiN(C₂F₅SO₂)₂, LiCF₃SO₃, LiC(CF₃ SO₂)₃, LiPF₄(CF₃)₂, LiPF₃(CF₃)₃,LiPF₃(iso-C₃F₇)₃, LiPF₅(iso-C₃F₇). The overall salt concentration may bebetween about 0.3M and 2.5M or, more specifically, between about 0.7Mand 1.5M, for example about 1.0M.

The electrolyte formulations described above may be used in cellsfabricated with various high capacity active materials. Some examples ofthese materials include various silicon containing materials (e.g.,crystalline silicon, amorphous silicon, other silicides, silicon oxides,sub-oxides, oxy-nitrides). Other examples include tin-containingmaterials (e.g., tin, tin oxide), germanium, carbon-containingmaterials, a variety of metal hydrides (e.g., MgH₂), silicides,phosphides, and nitrides. Additional examples include carbon-siliconcombinations (e.g., carbon-coated silicon, silicon-coated carbon, carbondoped with silicon, silicon doped with carbon, and alloys includingcarbon and silicon), carbon-germanium combinations (e.g., carbon-coatedgermanium, germanium-coated carbon, carbon doped with germanium, andgermanium doped with carbon), and carbon-tin combinations (e.g.,carbon-coated tin, tin-coated carbon, carbon doped with tin, and tindoped with carbon). More generally, a high capacity active material mayinclude silicon, tin, germanium, and aluminum as well as their alloysand other compounds. High capacity active materials are defined hereinas active materials with theoretical lithiation capacities of at leastabout 700 mAh/g.

The high capacity active material may be in various forms, such as auniform thin layer, particles bound to the substrate with the polymericbinder, substrate rooted nanostructures, and other forms and structures.In certain embodiments, high capacity active materials are formed assubstrate rooted nanostructures. These nanostructures may be physicallyand conductively attached to a conductive substrate, which may serve asa current collector for this electrode. The physical attachment may bemore than a simple mechanical contact, which might result, for example,from coating a binder with discrete nanostructures onto the substrate.In some embodiments, the physical attachment results from fusion of thenanostructures to the substrate or deposition of the nanostructures orsome portion of the nanostructures directly onto the substrate, forexample, using chemical vapor deposition (CVD) techniques or, even more,specifically using vapor-liquid-solid CVD growth. In yet anotherexample, physical attachment results from ballistic impalement of thenanostructures onto the substrate. In certain embodiments, physicalattachment includes certain forms of metallurgical bonds, such as aformation of alloys of two bonded materials (e.g., silicides).

Nanostructures may be rooted to the substrate at random locations on thestructure's profiles (randomly rooted), or rooted preferentially at someparticular location on the nanostructures (non-randomly rooted).Examples of non-randomly rooted nanostructures include terminally rootednanostructures (e.g., nanowires and nanorods) and medially rootednanostructures preferentially affixed to the substrate at medialpositions (rather than a terminal position). In particular examples,nanostructures are nanowires with an aspect ratio of at least about 4,or, more specifically, at least about 10. Nanowires may be between about10 nanometers and 1000 nanometers in diameter and between about 1micrometer and 100 micrometers in length.

In specific embodiments, an electrolyte containing DMPC and FEC is usedin a rechargeable cell having a silicon-based electrode, or morespecifically, substrate-rooted silicon nanowires. The electrolyte may beabout 5% by weight of FEC and about 10% by weight of DPMC added to ECand DMC present at equal weight ratios. In other embodiments, a weightratio of FEC may be about 10%, while a weight ratio of DPMC may be 2%.In yet other embodiments, DPMC may be presented at a weight ratio of 5%,and FEC may be present at a weight ratio of about 10% FEC. In certainembodiments, a lithium ion battery assembled with such electrolytesexhibits an average Coulombic efficiency of at least 99.8% after 100cycles during a cycling test. The Coulombic efficiency value may beaveraged over a predetermined number of cycles (e.g., five cycles or tencycles) to avoid the influence of noise and other unexpected variations.The cycling test may involve charging the lithium ion battery to 1050mAh/g at C/2 and discharging to 900 mV at C/2 rate in each cycle.

Various electrolyte formulations were tested using coin type (2032 size)half-cells with silicon containing test electrodes and lithium metalcounter electrodes. The working electrodes were constructed bydepositing a layer of silicon over a template structure containingnickel. These electrodes were combined with a 0.75 millimeter thicklithium foil and 25 micrometer thick separator. The test cells werefilled with different electrolyte formulations. All formulationsincluded 1M LiPF₆ and equal weight ratios of EC and DMC. Someformulations also included pyrocarbonates and/or fluorinated carbonates.One formulation that did not include any pyrocarbonates or fluorinatedcarbonates was used as a base. The cells were subjected to one formationcycle that included charging to 1400 mAh/g at the rate of C/20 followedby discharging to 900 mV at the same rate, which was not a part of thetest data. The cells were then tested using multiple operating cycles.In each operating cycle, the cells were charged to 1050 mAh/g at therate of C/2 and discharged to 900 mV at the same rate.

FIG. 1 illustrates Coulombic efficiency data for the first 350 operatingcycles. All lines were constructed using a 5-point smoothing techniqueto eliminate variations of individual cycles and to better reflect theoverall coulombic efficiency trends. The lines represent Coulombicefficiencies of six cells tested with different concentrations of DPMCand FEC added to the base electrolyte.

Line 101 corresponds to a cell built with a base electrolyte that didnot have any DPMC or FEC additives. This electrolyte formulation hasbeen successfully used in graphite-based cells showing Coulombicefficiencies of about 100%. However, when this electrolyte formulationwere tested with cells containing silicon based electrodes, the siliconbased cells performed much worse in terms of Coulombic efficiencies thanthe graphite-based cells. Specifically, the Coulombic efficiency of thetested silicon based cells started to decrease rapidly after only 20cycles and fell below 99.5% within 40 cycles.

Line 102 corresponds to a cell with 2 weight % of DMPC added to the baseelectrolyte. No FEC additives were used in this formulation. TheCoulombic efficiency improved compared to the additive-free formulationrepresented by line 101, but it still dropped rapidly and fell below99.5% within 60 cycles.

Line 103 corresponds to a cell built with 10 weight % of FEC added tothe base electrolyte. No DMPC additives were used in this formulation.The Coulombic efficiency of this cell was much better than that of thecell with the base electrolyte represented by line 101. For example,after 50 cycles, the Coulombic efficiency of this cell was still about99.9%, while the Coulombic efficiency of the cell with the baseelectrolyte was already below 99%. The coulombic efficiency eventuallyfell below 99.5% before the 200^(th) cycle.

Lines 104, 105, and 106 correspond to cells built with modifiedelectrolytes that contained both FEC and DMPC additives in differingconcentrations. Line 104 corresponds to a cell built with an electrolytecontaining 10 weight % of FEC and 2 weight % of DMPC. Line 105corresponds to a cell built with an electrolyte containing 10 weight %of FEC and 5 weight % of DMPC, while line 106 corresponds to a cellbuilt with an electrolyte containing 5 weight % of FEC and 10 weight %of DMPC. Using these combinations of DMPC and FEC improved the Coulombicefficiency even more. Specifically, the Coulombic efficiency did notdrop below the 99.5% level until after more than 200 cycles, which ismore than a ten-fold improvement over the base electrolyte.

When comparing lines 103 with lines 104 and 105, all of which correspondto electrolytes with 10 weight % of FEC, it can be seen that theaddition of DMPC to an electrolyte containing FEC helps to furtherimprove the Coulombic efficiency. Among the three electrolytescontaining both FEC and DMPC, the electrolyte with the highest DMPCcontent and lowest FEC content (corresponding to line 106) showed a muchslower initial rise in Coulombic efficiency during the first 50 cycles.This may indicate that higher concentrations of DMPC and lowerconcentrations of FEC, like the compositions represented by lines 104and 105, may be more optimized for use with silicon based electrodes.Without being restricted to any particular theory, it is believed thatthe Coulombic efficiency increase and stability when FEC and DMPC areused together is due to a complex synergistic effect between certainratios of FEC and DMPC that produce an SEI layer with uniquecharacteristics. FEC is widely believed to increase the fluorine contentin the SEI layer, and DMPC's effect may be due to increasing the lithiumcarbonate content in the SEI layer. Similar synergies have beenpreviously shown with FEC and other types of additives and salts incarbon-based cells.

Various high capacity active materials may be used to form an electrodefor use in a rechargeable cell. Some examples of these materials includevarious silicon containing materials, such as crystalline silicon,amorphous silicon, other silicides, silicon oxides, sub-oxides, andoxy-nitrides. Other examples include tin containing materials (such astin and tin oxide), germanium containing materials, carbon containingmaterials, a variety of metal hydrides (such as magnesium hydride),silicides, phosphides, and nitrides. Still other examples includecarbon-silicon combinations, such as carbon-coated silicon,silicon-coated carbon, carbon doped with silicon, silicon doped withcarbon, and alloys including carbon and silicon. Similar combinations ofcarbon and germanium, as well as similar combinations of carbon and tin,may be used. Various aluminum containing materials may be used as well.Overall, a high capacity active material may be defined as an activematerial with a theoretical lithiation capacity of at least about 700mAh/g. The theoretical capacity of the active material should bedistinguished from a deigned operating capacity of the electrode and/orcell. Generally, an operating capacity is a fraction of the theoreticalcapacity. For example, during operation of the cell, active material maynot be fully charged, discharged, or both. In a specific example, anelectrode containing silicon may be cycled between the operatingdischarge level (e.g., corresponding to the silicon lithiation state ofbetween about 500 mAh/g and 1500 mAh/g) and the operating charged level(e.g., corresponding to the silicon lithiation state of between about1500 mAh/g and 3000 mAh/g). For reference, the theoretical capacity ofsilicon is 4200 mAh/g.

Some high capacity active materials have poor electrical conductivity incomparison, for example, to graphite and other carbon based materials.Conductivity can be improved by introducing various conductive additivesinto structures containing high capacity active materials or providingconductive additive structures among the active material structures. Forexample, certain high capacity active materials may be doped duringtheir deposition (e.g., formation of nanostructure containing these highcapacity active materials) and/or during treatment of depositedmaterials. For the purposes of this document, any addition ofconductivity enhancement components into active material structures isreferred to as doping regardless of the concentration of theseconductivity enhancement components or methods of introducing thesecomponents. In certain embodiments, elements from the groups III and Vof the periodic table are used as conductivity enhancing components insilicon containing nanostructures. Specifically, silicon containingnanostructures may be doped with one or more elements from the groupconsisting of boron, aluminum, gallium, indium, thallium, phosphorous,arsenic, antimony, and bismuth. It has also been found that certainconductivity enhancement components improve charge transfer propertiesof the electrode. Other dopant atoms besides group III or V atoms may beemployed, such as sulfur and selenium. Doped silicon has a higherelectron or hole density in comparison with un-doped silicon (e.g.,Fermi level shifts into the conduction or valence band, resulting inhigher conductivity). In certain embodiments, silicon containingnanostructures are doped with lithium during fabrication of theelectrode and prior to electrochemical cycling of the fabricated cell.Lithium doping helps to compensate for losses of lithium during SEIformation. More than one dopant material may be used in the samenanostructure. Depending on the concentration of materials introducedinto silicon containing nanostructures, the resulting nanostructure maybe transformed into a semiconductor (concentration is between about 10¹⁴and 10¹⁹ atoms per centimeter cubed), a highly doped metalizedconductive silicon (concentration is between about 10¹⁹ and 10²¹ atomsper centimeter cubed), or a silicon alloy (concentration is greater thanabout 10²¹ atoms per centimeter cubed). A higher concentration isusually desirable for higher conductivity.

Various methods may be used to introduce dopants into thenanostructures. For example, a gas phase doping involves introducingdopant-containing precursors together with base material precursors,such as silane for silicon nanostructures. Relative flow rates ofprecursors may vary during deposition to achieve dopant concentrationgradients within the nanostructures. For example, a mixture of hydrogen,silane, and about 10 ppm of phosphine may be flowed into the depositionchamber. The silane gas decomposes at catalyst sites and forms siliconwires. The phosphine similarly decomposes and leaves phosphorus thatincorporates into the silicon nanostructures as a dopant by replacingsilicon in some lattice sites. Another method for doping involvesspin-on coating. For example, an organic polymer containing dopants maybe coated over a layer of deposited nanostructures. The coatednanostructures are then baked at between about 200° C. and 600° C. forbetween about 20 and 30 minutes. The organic polymer decomposes intogases that are removed from the baking chamber leaving dopant on thenanostructures. Some dopant may diffuse into the nanostructures. Anotherdoping method includes evaporating a dopant during the nanostructureformation and trapping some of the evaporated dopant in the newly formednanostructures. For example, aluminum and indium may dope siliconnanostructures using this method. A temperature range for evaporationmay be between about 150° C. and 700° C., depending on the material tobe evaporated.

Structures formed from active materials or, more specifically, from highcapacity active materials, may have various shapes and dimensionsdepending on compositions, crystallographic structures (e.g.,crystalline, amorphous), deposition process parameters, and many otherfactors. Shapes and sizes may also change during cycling. Sizes ofactive material structures may be characterized with a cross-sectionaldimension. For the purposes of this document, a cross-section dimensionis defined as a distance between the two most separated points on aperiphery of a cross-section that is transverse to the principaldimension, such as length. For example, a cross-section dimension of acylindrical nanowire is its diameter. A cross-section dimension of anirregular particle is the distance between its two most separatedcorners and/or surfaces. However, a cross-section dimension of a layeror a coating is an average thickness of this layer or coating.

High capacity active materials are generally formed into structures suchthat their cross-section dimensions are generally below the fracturelimits of these high capacity active materials. In certain embodiments,a cross-section dimension is between about 1 nanometer and 10,000nanometers. In more specific embodiments, a cross-section dimension isbetween about 5 nanometers and 1000 nanometers, and more specificallybetween 10 nanometers and 200 nanometers. These dimension ranges aregenerally applicable to silicon containing high capacity activematerials, such as amorphous or crystalline silicon.

High capacity active materials may be formed into various types ofnanostructures, which have cross-section dimensions less than 1,000nanometers (i.e., at least one nanoscale dimension). Some examples ofnanostructures include nanofilms that have a nanoscale dimension alongone axis, nanowires that have nanoscale dimensions along two axes, andnanoparticles that have nanoscale dimensions along all three axes. Forthe purposes of this document, nanowires are defined as structures thathave, on average, an aspect ratio of at least about four to distinguishthem from nanoparticles. In certain examples, the average aspect ratiomay be at least about ten, at least about one hundred, or even at leastabout one thousand. High capacity active materials formed into nanowirecan undergo substantial swelling without disrupting the overallelectrode structure. Nanowires also provide electrical and mechanicalconnections with the electrode, especially along their lengths, whichmay be extending substantially perpendicular to the conductive substrateand determine, at least in part, the thickness of the electrode.

Nanostructures containing high capacity active materials may be hollow.The cross-section profile of such hollow nanostructures includes voidregions surrounded by annular solid regions. An average ratio of thevoid regions to the solid regions may be between about 0.01 and 100,more specifically between about 0.01 and 10. The cross-section profileof hollow nanostructures may be substantially constant along theirprincipal dimensions, such as length of the nanotubes. Alternatively,the hollow nanostructures may be tapered along the principal dimension.Hollow nanostructures may have nanoscale wall thickness (e.g., less than1,000 nanometers). In the same or other embodiments, the averagecross-section dimension (e.g., the average diameter) of hollow nanotubesmay be less than 1,000 nanometers. In other embodiments, hollownanotubes may have a micrometer-scale cross-section dimension, whiletheir wall thickness is at the nanoscale.

The longest dimension of nanostructure is referred to as a principaldimension. For example, the principal dimension of nanowires is theirlength, which on average may be at least about 1 micrometer or, morespecifically, at least about 10 micrometers.

High capacity active materials may form a part of core-shellnanostructures, for example, and may be included as a part of a core,one or more shells, or both. FIG. 2A is schematic perspective view of acore-shell nanostructure 200 that includes a core 202, inner shell 204,and outer shell 206, in accordance with certain embodiments. It shouldbe understood that core-shell nanostructures may have any number ofinner shells (i.e., one or more such as up to fifty or, morespecifically, up to ten). A number, composition, and arrangement of suchshells may be driven by various functionalities of the nanostructures,such as electrical connections, mechanical support, high capacity, cyclelife, SEI layer formation, and many others. For clarity, FIG. 2Aillustrates one inner shell, and the description below is directed tothe one such shell. However, it should be understood that thisdescription is applicable to other configurations as well.

Generally, though not necessarily, core 202 and shells 204 and 206extend through the entire principal dimension, which is the longestdimension of the nanostructure 200 (e.g., its length (L)). For example,a core 202 and one or more shells 204 and 206 may share a substantiallycommon axis along the principal dimension. In certain embodiments, oneor more shells may be shorter than the principal dimension ofnanostructure 200. For example, an outer shell may extend less thanabout 90%, less than about 75%, or less than about 50% of the principaldimension. Further, a shell may completely cover a core or acorresponding inner shell. Alternatively, an outer shell may partiallycover an inner shell, leaving certain areas of the inner layer exposed.

FIG. 2B is schematic cross-section view of nanostructure 200, inaccordance with certain embodiments. Cross-sectional shapes ofnanostructures and each individual component generally depend oncompositions, crystallographic structures (e.g., crystalline,amorphous), sizes, deposition process parameters, and other factors.Shapes may also change during cycling.

Shells may be in a form of nested or concentric layers around a core,which may be in a form of a rod or wire. One shell (e.g., an innershell) may be surrounded by another shell (e.g., an outer shell)forming, for example, a set of concentric structures (e.g., cylinders)similar to the structure shown in FIG. 2B. In other embodiments (notshown), each layer of the nanostructure is a sheet that is rolled arounditself and other layers to form a spiral. For conciseness, both of theseembodiments are referred to as a core-shell structure. Not all shelllayers may be fully concentric to the core and/or other shell layers.For example, one or more shells may not cover the full angular extent ofthe core circumference or one of the inner shells. Such gaps may extendfully or partially along the length of the principal dimension.

Core-shell structures may have various shapes, such as nanorods/wireshapes, particle shapes (including spheres, ellipsoids, etc.), pyramidsrooted to a substrate, spider structures having multiple rods and/orparticles extending from a common connection point or region, and thelike. Further, the rods or other structures may have a non-linear shape,which includes shapes where the axial position bends or even assumes atortuous path.

Various components of core-shell structures may be characterized bytheir dimension in the cross-section plane as shown in FIG. 2B, such asvarious cross-sectional dimensions and shell thicknesses. In certainembodiments, an average cross-section dimension of the core (D) isbetween about 5 nanometers and 500 nanometers or, in more specificembodiments, between about 10 nanometers and 100 nanometers. Thisdimension will generally depend on the core materials (e.g.,conductivity, compressibility), thickness of the inner layer containingsilicon, and other parameters. For example, high rate batteryapplications may require a larger core to reduce an overall resistanceof the nanostructures. Generally, a cross-section dimension of the core(and thicknesses of shells further described below) does notsubstantially vary along the length of the nanostructure. However, incertain embodiments, the core (and possibly a resulting nanostructure)may be tapered or have a have variable cross-section dimension along thelength.

One or more inner shells may be characterized by their averagethicknesses (T1). When a high capacity active material is used to forman inner shell, the thickness values may be selected such that theactive material stays below its fracture stress level during cycling.Determining factors may include a crystallographic structure of the highcapacity active material (e.g., crystalline or amorphous), an averagecross-section dimension (D) of core 202, materials used for core 202 andouter shell 206, materials used for the inner shell 204, capacity andrate requirements, and other factors. The average thickness of the innerlayer may be between about 5 nanometers and 500 nanometers or, morespecifically, between about 10 nanometers and 100 nanometers.

Outer shell 206 may be designed to coat inner shell 204 and, forexample, to protect inner shell 204 from contacting an electrolyte. Inthese examples, an SEI layer is formed on a surface of outer shell 206and away from inner shell 204. Therefore, materials and othercharacteristics of outer shell 206 may be selected in such as a way thata more desirable SEI layer is formed, and this SEI layer remains duringcycling of the cell. Other functions and characteristics of outer shell206 may include high ionic conductivity and mechanical elasticity (e.g.,to reduce stress between swelling and contacting inner shell 204 and theSEI layer formed on the other side of outer shell 206). Further, outershell 206 may be used to provide electrical and/or mechanical contactsamong nanostructures in the electrode, to establish mechanical and/orelectrical contacts to the conductive substrate (if one is used), and/orother purposes. The thickness (T2) of outer shell 206 may be selected toprovide one or more functions listed above, and may be between about 1nanometer and 100 nanometers or, in more specific embodiments, betweenabout 2 nanometers and 50 nanometers.

In the same or other embodiments, an average principal dimension (L)(e.g., the average length for rood-like structure) of the core isbetween about 1 micrometer and 100 centimeters or, more specifically,between about 1 micrometer and 10 millimeters or, even morespecifically, between about 1 micrometer and 100 micrometers. Forexample, longer structures may be formed by electrospinning andpositioned generally parallel to the conductive substrate. Shorterstructures, such as shorter than 500 micrometers or, more specifically,shorter than 100 micrometers may be formed using growth rootedtechniques right on the substrate and extend substantially perpendicularto the substrate. More specifically, cores may be formed using growthrooted methods and may include conductive template materials, such asnickel silicides, or even high capacity active materials. Other lengthranges may include: between about 1 micrometer and 10 centimeters,between about 1 micrometer and 1 centimeter, or between about 1micrometer and 100 millimeters. Generally, the length of the overallstructure may be determined by the length of its core. However, incertain embodiments, the length of the overall structure may longer thanthe length of its core. In more specific embodiments, the core may bepartially or completely removed, resulting in hollow structures asfurther described below. It should be noted that some dimensionsdescribed below would change during electrochemical cycling of theelectrodes containing nanostructures. Unless otherwise noted, thesevalues correspond to newly deposited nanostructures before the initialcycling.

In certain embodiments, the core 202 is solid. For example, a core maybe a fiber (carbon, metal), a rod, a wire, or any other similar shape.In other embodiments, a core may be a hollow (e.g., tube-like) structureas. A hollow core may be formed from an initially solid core. Forexample, a solid core may be shrunk or partially removed to form ahollow core. In another embodiment, a hollow core may be formed bydepositing core materials around a template that is later removed. Incertain embodiments, a carbon single wall nanotube (SWNT) or multi-wallnanotube (MWNT) may serve as a core. The cross-sectional profile ofthese hollow nanostructures includes void regions surrounded by annularsolid regions. An average ratio of the void regions to the solid regionsmay be between about 0.01 and 100, or more specifically, between about0.01 and 10. The cross-section dimension of the hollow nanostructuresmay be substantially constant along the principal dimension (e.g.,typically the axis). Alternatively, the hollow nanostructures may betapered along the principal dimension. In certain embodiments, multiplehollow nanostructures may form a core-shell arrangement similar tomultiwall nanotubes.

As, mentioned, at least one inner shell typically includes a highcapacity material of a type further described below. However, a core andother shells may also contribute to an overall capacity of thenanostructure. In certain embodiments, the selection of materials anddimensions for each component of a nanostructure is such that one ormore inner shells containing high capacity materials provide at leastabout 50% of the overall nanostructure capacity or, in more specificembodiments, at least about 75% or at least about 90%.

Cores of core-shell structures may be used as a template for depositingone or more layers, and one of these layers may include a high capacityactive material. This template may be rooted to a conductive substrateto provide mechanical attachment and electrical conductivity. Thistemplate core may be grown from the conductive substrate, which mayinclude a source material layer used during growth of the templatesstructures.

FIG. 3 is a schematic representation of an electrode structure duringdifferent stages of its fabrication, in accordance with certainembodiments. Substrate 302 may be provided during an initial stage 301.Substrate 302 may include a base material and a metal source materialconsumed during formation of template cores. In certain materials, thebase material is also a source material. In other embodiments, twodifferent materials are used with a source material forming a top layerover the base. Substrate 302 may be then treated to form a surface 304that is suitable to form silicide nanostructures shown in stage 303. Forexample, treated surface 304 may have a certain degree of roughness andinclude specific nucleation sites for forming template structures.Surface 304 may also include masking materials. In certain applications,source materials and/or treated surfaces are formed on both sides of thesubstrate 302.

Silicide nanostructures 306 are then formed on the substrate 302 asshown in stage 305. In certain embodiments, silicide nanostructures 306have their ends rooted to the substrate 302. Silicide nanostructuresform a high surface area template that is used for depositing an activematerial. Finally, an active material layer 308 is deposited over thesilicide nanostructures 306 as shown in stage 307. Silicidenanostructures 306 can provide both mechanical support to the activematerial 308 and electrical connection to the substrate 302. While somecontact may exist between the active material and the substrate, it maynot be sufficient from a battery performance perspective.

A combination of silicide nanostructures 306 and active material 308 maybe referred to as an active layer 309. Generally, the active layer 309is adjacent to substrate 302 as shown in FIG. 3. Overall, active layer309 may be characterized by its height, which is typically close to theheight of the silicide template or, in specific embodiments, to thelength of the nanowires making this template. In certain embodiments, aheight of the active layer is between about 10 micrometers and 50micrometers or, more specifically, between about 20 micrometers and 40micrometers. Furthermore, active layer 309 may be characterized by itsporosity (e.g., at least about 25% or, more specifically, at least about50% or, even more specifically, at least about 75%), its capacity perunit area, and other characteristics.

Generally, the template material is highly electronically conductive andmechanically stable in the face of stresses experienced from expansionand contraction of active materials during cycling. Examples of suitabletemplate materials include metal silicides (e.g., copper silicides,nickel silicides, aluminum silicides), carbon, certain metal orsemiconductor oxides (e.g., zinc oxide, tin oxide, indium oxide, cadmiumoxide, aluminum oxide, titania/titanium dioxide, silicon oxides), andcertain metals (copper, nickel, aluminum). In particular embodiments,template structures are formed into nanowires and include silicides.Silicide nanowires may have a variable material composition along theirlengths (i.e., higher source material concentrations at the rooted(proximal) ends where more source material is available than near thefree (distal) ends of the nanowires). Depending on a source materialtype, this variability may be reflected in different morphological andstoichiometric phases of silicides. For example, nickel silicidenanowires may include one, two, or all three phases of nickel silicide(i.e., Ni2Si, NiSi, and NiSi2) It is believed that higher nickel contentphases form stronger bonds with nickel metal. Therefore, thisvariability may provide relatively strong adhesion of nickel silicidenanowires to the base layer and thereby reduce the contact resistance.The conductivity and lithium irreversibility of these different nickelsilicide phases also varies.

In certain embodiments, template nanowires may have wider bases (i.e.,be cone shaped nanowires). Cone shaped nanowires may result from, forexample, greater availability of the metal near the substrate/supportrooted ends of the nanowires. In certain embodiments, an averagediameter near the substrate/support rooted ends is at least about twicethat near the free end. That is, the bases of the nanowires may be largeenough to touch one another at the proximal ends on the surface of thesubstrate, but the distal tips are free and unconnected because of adecrease in diameter along the structure from the base to the tip. Inmore specific embodiments, a ratio of diameters between the proximal anddistal nanowire ends is at least about 4 or, more particularly, at leastabout 10. Wider bases may help to maintain adhesion to the substrate.

The template examples described above generally refer to templatesattached to planar substrates, such as metal foils. However, basestructures that support template structure may have other shapes, suchas meshes, foams, and particles. When particles are used as bases,template structures may be formed in all direction away from such bases.Such templates structures may be then coated with high capacity activematerials to form multidimensional electrochemically active structure.

FIG. 4 is a schematic two-dimensional (2D) representation of amultidimensional electrochemically active structure 400, in accordancewith certain embodiments. Multidimensional electrochemically activestructure 400 includes a support structure 402, which may be ananoparticle, nanowires, or any other type of nanostructure. Supportstructure 402 may include one or more source materials that are usedduring formation of nanowires 406. In certain embodiments, the entiresupport structure 402 is formed from the source material. In otherembodiments, support structure 402 includes an inert base and sourceshell, and the materials contained in the shell are used to formtemplate structures, such as nanowires.

Nanowires 406 are grown from support structure 402 and remain attachedto support structure 402 during later processing and use in the battery.Nanowires are defined as structures that have an aspect ratio of greaterthan one, typically at least about 2 and more frequently at least aboutfour. Nanowires 406 may extend into different directions (e.g., allthree dimensions) away from support 402. Such a template structure maybe fabricated by suspending support structure 402 and then partiallyformed template structures in a fluidized bed reactor. Such templatestructures may be later attached to the substrate by depositing highcapacity active materials over the structure and substrate or by using apolymer binder. In other embodiments, support structure 402 may bepositioned on a planar substrate and nanowires 406 are grown only intothe space over the substrate surface. In certain embodiments, attachmentto the substrate occurs during template structure formation and/orduring deposition of the high capacity active material over the templatestructure.

Nanowires 406 are coated with an active material, which forms an activematerial layer 408. In certain embodiments, the density of nanowires 406near support 402 prevents active materials layer 408 from coatingsupport 402. Keeping the active material area away from support 402 mayhelp to preserve connection between nanowires 406 and support 402.Multidimensional electrochemically active structure 400 may be thenarranged into a battery electrode, together with other such structures,to achieve any desirable thickness.

FIG. 5A is a plan view of a partially-assembled electrochemical cellthat uses electrodes described herein, according to certain embodiments.The cell has a positive electrode active layer 502 that is showncovering a major portion of a positive current collector 503. The cellalso has a negative electrode active layer 504 that is shown covering amajor portion of a negative current collector 505. Separator 506 isbetween the positive electrode active layer 502 and the negativeelectrode active layer 504.

In one embodiment, the negative electrode active layer 504 is slightlylarger than the positive electrode active layer 502 to ensure trappingof the lithium ions released from the positive electrode active layer502 by the active material of the negative electrode active layer 504.In one embodiment, the negative electrode active layer 504 extends atleast between about 0.25 millimeters and 5 millimeters beyond thepositive electrode active layer 502 in one or more directions. In a morespecific embodiment, the negative electrode active layer extends beyondthe positive electrode active layer by between about 1 millimeter and 2millimeters in one or more directions. In certain embodiments, the edgesof the separator 506 extend beyond the outer edges of at least thenegative electrode active layer 504 to provide the complete electronicinsulation of the negative electrode from the other battery components.

FIG. 5B is a cross-section view of an electrode stack 500 of thepartially-assembled electrochemical cell that uses electrodes describedherein, according to certain embodiments. There is a positive currentcollector 503 that has a positive electrode active layer 502 a on oneside and a positive electrode active layer 502 b on the opposite side.There is a negative current collector 505 that has a negative electrodeactive layer 504 a on one side and a negative electrode active layer 504b on the opposite side. There is a separator 506 a between the positiveelectrode active layer 502 a and the negative electrode active layer 504a. The separator sheets 506 a and 506 b serves to maintain mechanicalseparation between the positive electrode active layer 502 a and thenegative electrode active layer 504 a and acts as a sponge to soak upthe liquid electrolyte (not shown) that will be added later. The ends ofthe current collectors 503, 505, on which there is no active material,can be used for connecting to the appropriate terminal of a cell (notshown).

Together, the electrode layers 502 a, 504 a, the current collectors 503,505, and the separator 506 a can be said to form one electrochemicalcell unit. The complete stack 500 shown in FIG. 5B includes theelectrode layers 502 b, 504 b and the additional separator 506 b. Thecurrent collectors 503, 505 can be shared between adjacent cells. Whensuch stacks are repeated, the result is a cell or battery with largercapacity than that of a single cell unit.

Another way to make a battery or cell with large capacity is to make onevery large cell unit and wind it in upon itself to make multiple stacks.The cross-section schematic illustration in FIG. 6A shows how long andnarrow electrodes can be wound together with two sheets of separator toform a battery or cell, sometimes referred to as a jellyroll 600. Thejellyroll is shaped and sized to fit the internal dimensions of acurved, often cylindrical, case 602. The jellyroll 600 has a positiveelectrode 606 and a negative electrode 604. The white spaces between theelectrodes are the separator sheets. The jelly roll can be inserted intothe case 602. In some embodiments, the jellyroll 600 may have a mandrel608 in the center that establishes an initial winding diameter andprevents the inner winds from occupying the center axial region. Themandrel 608 may be made of conductive material, and, in someembodiments, it may be a part of a cell terminal. FIG. 6B shows aperspective view of the jelly roll 600 with a positive tab 612 and anegative tab 614 extending from the positive current collector (notshown) and the negative current collector (not shown), respectively. Thetabs may be welded to the current collectors.

The length and width of the electrodes depend on the overall dimensionsof the cell and thicknesses of the active layers and the currentcollectors. For example, a conventional 18650-type cell with 18 mmdiameter and 65 mm length may have electrodes that are between about 300and 1000 mm long. Shorter electrodes corresponding to lower rate/highercapacity applications are thicker and have fewer winds.

A cylindrical design may be used for some lithium ion cells, especiallywhen the electrodes can swell during cycling and thus exert pressure onthe casing. It is useful to use a cylindrical casing that is as thin aspossible while still being able to maintain sufficient pressure on thecell (with a good safety margin). Prismatic (flat) cells may besimilarly wound, but their case may be flexible so that they can bendalong the longer sides to accommodate the internal pressure. Moreover,the pressure may not be the same within different parts of the cell, andthe corners of the prismatic cell may be left empty. Empty pocketsgenerally should be avoided within lithium ions cells because electrodestend to be unevenly pushed into these pockets during electrode swelling.Moreover, the electrolyte may aggregate in empty pockets and leave dryareas between the electrodes, negatively affecting lithium ion transportbetween the electrodes. Nevertheless, for certain applications, such asthose dictated by rectangular form factors, prismatic cells areappropriate. In some embodiments, prismatic cells employ stacks ofrectangular electrodes and separator sheets to avoid some of thedifficulties encountered with wound prismatic cells.

FIG. 6C illustrates a top view of a wound prismatic jellyroll 620. Thejellyroll 620 includes a positive electrode 624 and a negative electrode626. The white space between the electrodes is the separator sheet. Thejelly roll 620 is enclosed in a rectangular prismatic case 622. Unlikethe cylindrical jellyrolls shown in FIGS. 6A and 6B, the winding of theprismatic jellyroll starts with a flat extended section in the middle ofthe jelly roll. In one embodiment, the jelly roll may include a mandrel(not shown) in the middle of the jellyroll onto which the electrodes andseparator are wound.

FIG. 7A illustrates a cross-section of a stacked cell that includes aplurality of cells (701 a, 701 b, 701 c, 701 d, and 701 e), each havinga positive electrode (e.g., 703 a, 703 b), a positive current collector(e.g., 702), a negative electrode (e.g., 705 a, 705 b), a negativecurrent collector (e.g., 704), and a separator (e.g., 706 a, 706 b)between the electrodes. Each current collector is shared by adjacentcells. A stacked cell can be made in almost any shape, which isparticularly suitable for prismatic batteries. The current collectortabs typically extend from the stack and lead to a battery terminal.FIG. 7B shows a perspective view of a stacked cell that includes aplurality of cells.

FIG. 8 illustrates a cross-section view of the wound cylindrical cell,in accordance with one embodiment. A jelly roll includes a spirallywound positive electrode 802, a negative electrode 804, and two sheetsof the separator 806. The jelly roll is inserted into a cell case 816,and a cap 818 and gasket 820 are used to seal the cell. In some cases,cap 818 or case 816 include a safety device. For example, a safety ventor burst valve may be employed to break open if excessive pressurebuilds up in the battery. Also, a positive thermal coefficient (PTC)device may be incorporated into the conductive pathway of cap 818 toreduce the damage that might result if the cell suffered a shortcircuit. The external surface of the cap 818 may be used as the positiveterminal, while the external surface of the cell case 816 may serve asthe negative terminal. In an alternative embodiment, the polarity of thebattery is reversed and the external surface of the cap 818 is used asthe negative terminal, while the external surface of the cell case 816serves as the positive terminal. Tabs 808 and 810 may be used toestablish a connection between the positive and negative electrodes andthe corresponding terminals. Appropriate insulating gaskets 814 and 812may be inserted to prevent the possibility of internal shorting. Duringfabrication, the cap 818 may be crimped to the case 816 in order to sealthe cell. However, prior to this operation, electrolyte (not shown) isadded to fill the porous spaces of the jelly roll.

A rigid case is typically required for lithium ion cells, while lithiumpolymer cells may be packed into a flexible, foil-type (polymerlaminate) case. A variety of materials can be chosen for the case. Forlithium-ion batteries, Ti-6-4, other Ti alloys, Al, Al alloys, and300-series stainless steels may be suitable for the positive conductivecase portions and end caps, and commercially pure Ti, Ti alloys, Cu, Al,Al alloys, Ni, Pb, and stainless steels may be suitable for the negativeconductive case portions and end caps.

Although the foregoing concepts have been described in some detail forpurposes of clarity of understanding, it will be apparent that certainchanges and modifications may be practiced within the scope of theappended claims. It should be noted that there are many alternative waysof implementing the processes, systems, and apparatuses. Accordingly,the present embodiments are to be considered as illustrative and notrestrictive.

What is claimed is:
 1. A lithium ion battery comprising: an electrodecomprising an electrochemically active material selected from the groupconsisting of a silicon containing material, a tin containing material,a germanium containing material, and an aluminum containing material; anelectrolyte comprising a lithium containing salt, a pyrocarbonate, andone or more fluorinated carbonate solvents; and a solid electrolyteinterphase (SEI) layer that includes fluorine, wherein the electrodecomprising an electrochemically active material includes core-shellnanostructures that include an inner shell and an outer shell; andwherein the SEI layer is formed on the outer shell, the nanostructureshaving at least one cross-sectional dimension between 1 and 1000 nm. 2.The lithium ion battery of claim 1, wherein the pyrocarbonate isselected from the group consisting of dimethyl pyrocarbonate, diethylpyrocarbonate, dipropyl pyrocarbonate, dibutyl pyrocarbonate, andethylmethyl pyrocarbonate.
 3. The lithium ion battery of claim 1,wherein the pyrocarbonate is dimethyl pyrocarbonate.
 4. The lithium ionbattery of claim 1, wherein the electrochemically active materialcomprises elemental silicon.
 5. The lithium ion battery of claim 1,wherein the electrochemically active material comprises a silicon alloy.6. The lithium ion battery of claim 1, wherein the electrode comprisesnanostructures, the nanostructures comprising the electrochemicallyactive material.
 7. The lithium ion battery of claim 6, wherein thenanostructures comprise nanowires.
 8. The lithium ion battery of claim7, wherein an outer surface of the nanowires comprise elemental silicon,silicon oxide, silicon alloy, or silicide.
 9. The lithium ion battery ofclaim 1, wherein the electrochemically active material comprisessilicon, wherein the lithium ion battery exhibits an average Coulombicefficiency of at least 99.8% after about 100 cycles during a cyclingtest; and wherein the cycling test comprises charging the lithium ionbattery to at least about 1050 mAh/g at a rate of at least about C/2 anddischarging to 900 mV versus lithium metal at a rate of at least aboutC/2 in each cycle.
 10. The lithium ion battery 1, wherein theelectrolyte comprises one or more pyrocarbonates that have a totalconcentration in the electrolyte of less than about 50% by weight. 11.The lithium ion battery 1, wherein the electrolyte comprises one or morepyrocarbonates that have a total concentration in the electrolyte ofbetween about 1% and 10% by weight.
 12. The lithium ion battery of claim1, wherein the one or more fluorinated carbonate solvents are selectedfrom the group consisting of mono-fluoroethylene carbonate,fluoropropylene carbonate, difluoroethylene carbonate, andfluoromethylethylene carbonate.
 13. The lithium ion battery of claim 1,wherein the one or more fluorinated carbonate solvents comprisesmono-fluoroethylene carbonate (FEC).
 14. The lithium ion battery ofclaim 1, wherein the one or more fluorinated carbonate solvents have atotal concentration in the electrolyte of less than about 50% by weight.15. The lithium ion battery of claim 14, wherein the one or morefluorinated carbonate solvents have a total concentration of less thanabout 10% by weight.
 16. A lithium ion battery electrolyte for use in alithium ion battery containing high capacity active materials, thelithium ion battery electrolyte comprising: a lithium containing salt; asolvent substantially free from linear carbonates; dimethylpyrocarbonate (DMPC); and one or more fluorinated carbonate solventssubstantially free from linear carbonates, wherein the DMPC is presentin the lithium battery electrolyte at a concentration between 5% and 10%by weight and wherein the one or more fluorinated carbonates are presentin the lithium ion battery electrolyte at a concentration of betweenabout 1% and 10% by weight.
 17. The lithium ion battery electrolyte ofclaim 16, wherein at least one of the one or more fluorinated carbonatesolvents is selected from the group consisting of mono-fluoroethylenecarbonate, fluoropropylene carbonate, difluoroethylene carbonate, andfluoromethylethylene carbonate.
 18. The lithium ion battery electrolyteof claim 17, wherein the one or more fluorinated carbonate solventscomprise mono-fluoroethylene carbonate (FEC).
 19. The lithium ionbattery electrolyte of claim 16, wherein the solvent is selected fromthe group consisting of ethylene carbonate (EC), dimethyl carbonate(DMC), diethyl carbonate (DMC), and/or ethyl-methyl carbonate (EMC). 20.The lithium ion battery electrolyte of claim 16, wherein the lithiumcontaining salt is selected from the group consisting of lithiumhexafluorophosphate (LiPF₆), lithium hexafluoroarsenate monohydrate(LiAsF₆), lithium perchlorate (LiClO₄), lithium tetrafluoroborate(LiBF₄), lithium bis(oxalato)borate (LiBOB), lithiumoxalyldifluoroborate (LiODFB), LiPF₃(CF₂CF₃)₃ (LiFAP), andLiBF₃(CF₂CF₃)₃ (LiFAB).
 21. The lithium ion battery electrode of claim1, wherein the electrolyte consists of one or more carbonate solvents,one or more lithium-containing salts, and one or more additives, the oneor more additives including the pyrocarbonate.
 22. The lithium ionbattery electrode of claim 1, wherein the electrolyte comprises one ormore pyrocarbonates that have a total concentration in the electrolyteof between 5% and 10% by weight.
 23. The lithium ion battery of claim 1,wherein the solvent is substantially free from linear carbonates.